: Heart transplant rejection is accompanied by increased oxygen free radical activity. These oxygen free radicals degrade membranes by lipid peroxidation, releasing volatile organic compounds (VOCs) which are excreted in the breath. Hence, breath VOC analysis may detect markers of rejection. Breath VOCs were analyzed in 28 heart transplant patients prior to endomyocardial biopsy. Ten volatile VOCs (including pentane) were significantly increased in those patients with histological evidence of rejection. A discriminant analysis model employing these VOCs identified rejection with 83.3% sensitivity and 100% specificity. The investigator hypothesizes that breath testing for VOCs may provide a useful new early diagnostic test for heart transplant rejection. The experimental plan will be to: 1) to perform breath VOC analysis in 200 cardiac transplant patients undergoing endomyocardial biopsy, 2) identify significant VOC markers of rejection and combine these in a model using discriminant analysis and 3) determine the sensitivity, specificity and predictive value of the breath test. The long term aim of the research is to determine if breath testing can detect heart transplant rejection and reduce the need for routine endomyocardial biopsy. PROPOSED COMMERCIAL APPLICATION: Potential commercial application not available.